Hi Stef1342,
Sorry to hear about the playback issues using the Scarlett 18i8 with Pro Tools, but we are more than happy to help. First, please take the time to go through the appropriate Avid optimization guide for your system found in the following link:
http://avid.force.com/pkb/articles/e...oting/en367983
After implementing the listed steps, please proceed with closing any additional background applications and verify the device is not selected in the Windows sound properties. When launching Pro Tools, it is recommended that the program is ran as an administrator. Once opened, please test further to verify if issues are still present.
If you are still encountering difficulty, please feel free to contact us through the following link so we may investigate further:
https://support.focusrite.com/hc/en-gb/requests/new